The present invention relates generally to turbo-machinery that must be both powerful and that must carry structural loads in aircraft applications. More specifically, the present invention relates to apparatus and methods relating to a scroll housing for use with a compressor.
In aviation applications, it is necessary to provide compressed air from the aircraft engines to the aircraft. Gas turbine engine aircraft may utilize an auxiliary power unit (APU) to provide air both when an aircraft is on the ground and when it is in flight. Air can be taken from an engine to pressurize or to otherwise condition the cabin air or, for example, to cool avionics equipment. In these aviation applications, there is a constant drive to both improve performance and to reduce the weight of components.
In aviation applications, a centrifugal compressor can be used to compress air. In these cases, the compressor discharge scroll must also be capable of supporting a variety of loading stresses that will occur. Specifically, a compressor scroll must be able to support dynamic loading from the aircraft environment, and pressure loading from the pressurization of air that occurs from the compressor itself.
U.S. Pat. No. 4,378,194 (the '194) shows one method of supporting these loads. In the '194 patent, there can be carcass loads that will develop between the forward and aft sections of the aircraft engine. The scroll housing surrounding the centrifugal compressor impeller is designed with a wall thickness that is great enough to handle these stress loads as well as the stress that will develop from air pressurized within the scroll. This is a common approach and one that works in some applications, although the weight of the heavy scroll wall and space restrictions limits the applicability of this approach.
U.S. Pat. No. 3,963,369 discloses another centrifugal compressor that will withstand the stresses in an aviation environment. Through-bolts are installed through the diffuser, which surrounds the impeller. The bolts are laid out in a circular pattern surrounding the impeller. The through-bolts serve to transmit engine carcass loads through the compressor housing. The greater the design loads, the more through-bolts are used to carry the load. Bolts passing through the diffuser work in some applications, but their use is generally limited to applications in which their presence does not limit performance. As performance requirements push the need for greater airflow and reduced weight, through-bolts become a limiting factor in the design. The through-bolt design also has the problem of increased part count and tolerance buildup associated with the increased part count. Tolerance build up can occur when multiple parts must fit together in an assembly such as the through bolt compressor housing which has a plurality of through-bolts as well as at least two housing halves to be assembled.
FIG. 1 shows a portion of a prior art turbo-machine including a scroll housing 100 designed so that all structural loads, including engine carcass loads, are carried solely by the scroll housing outer wall. In this design, the load path is contained in the scroll housing wall and does not cross the airflow path. This design requires a scroll housing wall thick enough to support all loading on the scroll housing 100. A direct load path S2 is required to maintain wall stresses comprising primarily tensile and shear components. In this prior art, as scroll wall 102 curvature between flanges 104 and 106 increases, bending stress becomes more predominant, and since bending is a less efficient means of supporting loads, material must be added to the scroll wall 102 in order to maintain adequate strength and stiffness, thus adding unwanted weight to the scroll housing 100. The need to keep the scroll wall 102 relatively flat limits the size of the scroll flow path. As the scroll wall 102 gets more of a bulge, it must get thicker to remain stiff enough, thus the weight increases. So, minimizing weight leads to minimizing curvature, which puts limits on scroll size relative to the flange diameters of the scroll. If the size of the flanges 104, 106 are increased to minimize curvature, then weight and installation are adversely impacted. A scroll wall 102 is thick enough to carry all structural loading transmitted between a forward flange 104 and an aft flange 106. The load path S2 is entirely contained within scroll wall 102. The scroll vanes 110 guide airflow A2 but carry no structural load and are outside the load path S2. This design also requires at least two housing piece parts, leading to greater weight, cost, and tolerance buildup associated with fitted parts.
FIG. 2 shows a portion of another prior art turbo-machine including a scroll housing 200 where bolts, passing through the diffuser, carry all the structural loads including engine carcass load. The through-bolt design requires diffuser vanes that are wide enough to accommodate the through-bolts. In many gas turbine engines the diffuser vanes are not large enough to accommodate the through-bolts. Through-bolts 202 carry all structural load, indicated by load path S3, between a forward flange 204 and an aft flange 206. Scroll vanes 210 guide air and can carry some of the pressure load S4 generated by airflow A3 within scroll housing 200 itself. Scroll vanes 210 are outside the structural load path S3, and the scroll wall 212 will carry no structural load.
As can be seen, there is a need for an improved scroll discharge housing for a centrifugal compressor and a method of making the scroll housing for use with an impeller. There is a need for a compressor discharge scroll housing design that maximizes performance while minimizing weight and part count. There is also a need for a compressor discharge scroll housing that allows for optimum air flow performance while being designed to withstand essentially all the stresses associated with pressure and engine carcass loads.